Magnetoptic head for recording/erasing information using a cylindrical hollow permanent magnet

ABSTRACT

To reduce the size and thickness of a magnetooptic head, a single hollow cylindrical permanent magnet magnetized in the radial direction thereof is arranged at the center of a condenser lens. The axial component of the magnetic flux is available for an external magnetic field required for recording and erasing operations, while the radial component of the magnetic flux is available for a magnetic field required for focusing operation in cooperation with a cylindrical focusing coil arranged coaxially with the cylindrical magnet.

This application is a division of application Ser. No. 07/077,851, filedJuly 27, 1987 now U.S. Pat. No. 4,868,802.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetooptic head which can record,read, and erase information on or from a magnetooptic disk.

2. Description of the Prior Art

So far, various optical disk memories have been developed as memoryunits of higher density and large capacitance. In parallel to theresearch and development of the disks, optical disk drives have beendeveloped, of course. However, the higher density and larger capacitanceof the disk memory are greatly dependent upon the optical head memory.In other words, it is no exaggeration to say that the head decides thevalue of the memory unit.

The optical head is disclosed in some Patent Applications. For instance,Japanese published Unexamined Patent Appli. (Kokai) No. 58-12145discloses a head in which a permanent magnet is fixed to a condenserlens and coils are mounted on a fixed base near the permanent magnet todrive the magnet in at least two directions for providing tracking andfocusing control, independently. Further, Japanese Patent disclosure(Kokai) No. 59-221839 discloses a head in which coils are bonded to acondenser lens and a permanent magnet is fixed to a fixed base to drivethe coils in at least two directions for providing tracking and focusingcontrol, independently.

In the above-mentioned write-once type optical disk, information oncewritten is not erasable or overwritable. However, it is of coursedesirable to erase unnecessary information from a disk to effectivelyutilize the optical disk.

Therefore, an overwritable magnetooptic disk memory has been developedand highlighted. The disk memory of this type will be describedhereinbelow in brief.

To record information signals on the magnetooptic disk, a 1 μm-dia.laser spot light is focused locally on the disk under an externalmagnetic field of 500 Oe or less to raise local temperature on themagnetic film and thereby to change the local magnetization direction inparallel to the external magnetic field. In this case; a pit of a 0.6 to1 μm width and a 1 to 2 μm length is formed on the disk. To eraseinformation signals from the disk, another external magnetic fieldopposite to the recording external field in direction is applied to thedisk. To read information signals from the disk, the disk is irradiatedwith a linearly polarized laser beam to detect a difference inpolarization direction of reflected or transmitted light between areasat which magnetization direction is inversed and those at which notinversed.

As described above, in order to record, read and erase information to orfrom the disk, external magnetic fields are necessary in addition to alaser beam for locally heating the magnetic film.

FIG. 1(A) shows an example of prior-art external magnetic fieldgenerating means disclosed in the two already-mentioned Japanese patentApplications, in which an optical head OH and a permanent magnet PM (oran electromagnet) are disposed on both sides of a disk D in opposingpositional relationship to each other.

In the above arrangement, it is desirable that the intensity of theexternal magnetic field is substantially constant in both informationrecording and erasing operations. However, whenever the disk rotates,since the disk is subjected to dynamic axial runout, the distancebetween the disk D and the permanent magnet PM changes, so that theintensity of the external magnetic field on the disk fluctuates.

The fluctuations of the external magnetic field intensity causevariations in recorded pit size or in erased track width, thus resultingin drawbacks such that information signal quality, that is, disk drivereliability is deteriorated.

To overcome the above fluctuations, although it is possible to increasethe distance between the disk and the external magnetic field generatingmeans, the generating means may become large in size and another problemwith heat generation may arise in the case of an electromagent.

To overcome the above-mentioned problem, Japanese Published UnexaminedPatent Applic. (Kokai) No. 61-96540 discloses a head including a movablemagnet as shown in FIG. 1(B). In this head, a cylindrical magnet MG isfixed to a condenser lens CL for focusing a laser beam LB onto a surfaceof a disk D, coaxially with the condenser lens CL. The magnet MG isadjustably moved to or away from the disk D by a focusing coil FC,together with the condenser lens.

In the above-mentioned prior art head, however, there exists anotherproblem in that another tracking coil and another tracking magnet shouldbe arranged within the same head, thus resulting in a relativelycomplicated mechanism, an increase in size and thickness of the head, aninterference in magnetic field between focusing magnet and trackingmagnet.

In the optical disk device of large capacity and high density, it isindispensable to implement focusing control and tracking control for thecondenser lens. For doing this, a tracking guide groove called pregrooveis formed on the disk in the concentric or spiral fashion with a trackpitch of about 1.6 to 2 μm. The condenser lens is controllably drivenalong the pregroove (in the tracking direction) on the basis of atracking signal detected from the groove.

In the above-mentioned track access operation of the laser beam, thehead is usually controlled in accordance with two, coarse and fine,operations. That is, when the access distance for tracking control is 50to 60 μm or more, the head is coarsely moved by an external actuatorsuch as a linear motor; while when the access distance is 50 to 60 μm orless, only the condenser lens is finely moved by an internal actuatorhoused within the head.

In the coarse track access operation of the above two-stage servotracking system, the condenser lens is locked at a predetermined(central) position within the head by passing a current through thetracking coils in order to prevent vibrations of the condenser lenscaused when the head is stopped near a designated track and to enter thesucceeding fine track access operation immediately. On the other hand,in the fine tracking operation, the condenser lens is located at thecentral position within the head. In summary, the tracking operation canbe achieved by a fine tracking actuator provided inside the head and acoarse linear actuator provided outside the head in combination forproviding a high speed track access operation. Therefore, it isnecessary to accurately detect the position of the condenser lensrelative to the head especially for two stage tracking servosystem.

To detect the condenser lens position, conventionally two, reflectedlight and transmitted light, detection methods have been used.

FIG. 1(C) shows a reflected light detection method, by which lightemitted from a light emitting diode LED is guided through a lens LS,reflected by a mirror M attached on a holder of a condenser lens CL, andthen received by a two-element photodetector PD to detect unbalancebetween the two. In this case, the position of the condenser lens CL canbe detected on the basis of a differential output between two outputsignals generated from the two-element photodetector PD.

FIG. 1(D) shows a transmitted light detection method, by which lightemitted from an LED is guided through a lens LS, transmitted through aslit SL and then received by a two-element photodetector PD to detect anunbalance between the two caused when the slit SL moves.

In the prior-art condenser lens position detecting apparatus, thereexist various drawbacks such that optical elements are required to belocated at accurate positions relative to the lens holder; the number ofparts is large; the movable condenser lens is unbalanced in weightbecause some additional elements are attached onto the lens holder, thusresulting in an inclination of the lens actuator or unstability thereof.

SUMMARY OF THE INVENTION

With these problems in mind, therefore, it is the primary object of thepresent invention to provide a magnetooptic head small in size, thin inthickness and high in information signal reliability.

To achieve the above-mentioned object, a magnetooptic head forrecording/erasing information on/from a recording medium by irradiatingthe recording medium with a laser beam within magnetic field accordingto the present invention comprises: (a) condenser lens unit for focusingthe beam on the recording medium; (b) magnetic field generating unitfixedly arranged outside of the condenser lens unit, for generating arecording/erasing external magnetic field in an axial direction of saidcondenser lens unit and focusing and tracking actuator magnetic fieldsin a radial direction of said condenser lens means; (c) at least twofocusing coil unit arranged within the radial-direction magnetic fieldin symmetrical positional relationship with respect to a center of saidcondenser lens unit, for moving the condenser lens unit and saidmagnetic field generating unit together in an axial direction of therecording medium, for providing an automatic focusing servo operation ofthe laser beam; (d) at least two tracking coil unit also arranged withinthe radial-direction magnetic field in symmetrical positionalrelationship with respect to the center of said condenser lens unit, formoving the condenser lens unit and said magnetic field generating unittogether in a radial direction of the recording medium, for providing atracking servo operation of the laser beam; and (e) elastic unit forsupporting the condenser lens unit and said magnetic field generatingunit together so as to be, movable relative to said focusing andtracking coil unit in the axial and radial directions of said recordingmedium.

Further, it is preferable to further provide the head with at least onemagnetic field intensity detecting unit arranged near said magneticfield generating unit, for detecting direction and displacement of saidmagnetic field generating unit relative to said focusing and trackingcoil unit to lock said condenser lens unit and to move said head itselffor coarse track access operation.

The magnet field generating means is a permanent magnet which can applyan external recording or erasing magnetic field, focusing and trackingoperation magnetic fields, and condenser lens position detectingmagnetic field, all together, thus reducing the size and thickness ofthe head and improving the reliability of the head without beingsubjected to magnetic field interference.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the magnetooptic head according to thepresent invention will be more clearly appreciated from the followingdescription of the preferred embodiments of the invention taken inconjunction with the accompanying drawings in which like referencenumerals designate the same or similar elements or sections throughoutthe FIGURES thereof and in which:

FIG. 1(A) is a perspective view showing a prior-art optical head and apermanent magnet;

FIG. 1(B) is a diagrammatical view showing another prior-art opticalhead;

FIG. 1(C) is a perspective view showing a prior-art method of detectinga head position;

FIG. 1(D) is a perspective view showing another prior-art method ofdetecting a head position;

FIG. 2(A) is a block diagram showing a head driving system of themagnetooptic head according to the present invention;

FIG. 2(B) is an illustration for assistance in explaining the erasingand recording operations of the head according to the present invention;

FIG. 3(A) is a top view showing a first embodiment of the head of thepresent invention;

FIG. 3(B) is a side view showing the head shown in FIG. 3(A);

FIG. 4(A) is a view for assistance in explaining the operation of thehead of the present invention;

FIG. 5(A) is a top view showing a first modification of the firstembodiment;

FIG. 5(B) is a side view showing the head shown in FIG. 5(A);

FIG. 6 is a perspective view showing a second modification of the firstembodiment;

FIG. 7(A) is a top view showing a second embodiment of the headaccording to the present invention;

FIG. 7(B) is a side view showing the head shown in FIG. 7(A);

FIG. 8(A) is a perspective view showing a third embodiment of the headaccording to the present invention;

FIG. 8(B) is a top view showing the head shown in FIG. 8(B);

FIG. 9(A) is an illustration showing a Hall element;

FIG. 9(B) is a differential circuit;

FIG. 9(C) is a graph representing a linear relationship betweendifferential output and lens displacement;

FIG. 10 is a perspective view showing a first modification of the thirdembodiment;

FIG. 11(A) is a perspective view showing a second modification of thethird embodiment;

FIG. 11(B) is a perspective view showing a third modification of thethird embodiment;

FIG. 12(A) is a top view of an example of head to which the secondembodiment is partially adopted;

FIG. 12(B) is a side view of the head shown in FIG. 12(A);

FIG. 13(A) is a top view of another example of head to which the secondembodiment is partially adopted; and

FIG. 13(B) is a side view of the head shown in FIG. 13(A).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2(A) is a block diagram showing a magnetooptic disk drive includingmagnetooptic heads as an embodiment of the present invention, with amagnetooptic disk as a recording medium.

A magnetooptic disk (referred to as a disk) 1 is mounted on a turntable3 driven by a motor 2 rotating at a constant rotational speed under thecontrol of a motor controller 4. The numeral 5a denotes a first(erasing) magnetooptic head, and 56 denotes a second (recording)magnetooptic head. Each of these heads comprises an optical devicesincluding a laser diode, optical sensors, various optical elements(condenser lens) etc., to irradiate the disk 1 with a laser beam fordetection of tracking control signal, focusing control signal andreadout information signal; and a permanent magnet for generatingexternal magnetic field required for recording and erasing operations,all elements moving together with a condenser lens. Further, as alreadydescribed, the direction of external magnetic field generated by thefirst head 5a is opposite to that of the external magnetic fieldgenerated by the second head 5b on the magnetooptic disk 1. These twoheads 5a and 5b are finely controlled by first and second headcontrollers 6a and 6b, independently, and additionally coarsely by firstand second linear motors 8a and 8 b controlled by first and secondlinear motor controllers 7a and 7b, independently, along disk radialdirection.

The apparatus shown in FIG. 2A is provided with recording mode andreadout mode. In operation of recording or readout mode, arecord/readout mode designation signal, a record/readout selectoraddress, a record/readout bit number, record information signals, etc.are transferred between an external system (not shown) and amagnetooptic disk controller 10 via an interface 9. A sector addresscontroller 11 controls the head 5a or 5b so as to access a designatedsector. A record mode controller 12 or a readout mode controller 13executes each designated mode in unit of sector.

With reference to FIG. 2(B), the overwrite operation of the magnetooptichead will be described. An information signal recording area RA of thedisk 1 extends from near the center thereof in spiral fashion, andfurther each annular track area is divided into N sectors Sectoraddresses are determined in sequence beginning from the innermostcircumference track to the outermost circumference track.

The first erasing magnetooptic head 5a moves in the radial direction ofthe disk 1 as shown by the arrow A, while the second recordingmagnetooptic head 5b moves in the radial direction of the disk 1 asshown by arrow B.

The two heads are disposed in symmetrical positional relationship withrespect to the center of the disk, and erases and then overwritesinformation from and to the disk 1. In more detail, the two heads makeaccess to a track to be overwritten in response to a sector address Whenthe disk 1 rotates clockwise as shown in FIG. 2(B), the first erasinghead 5a first erases some divided sector areas. Thereafter, when theerased sector area reaches the second recording head 5b, the head 5boverwrites new information on the erased area.

Since two heads 5a and 5b are used for overwriting information, therecording time can be reduced by half, as compared with the case whereinformation is overwritten by operating a single head, thus improvingthe throughput time of the disk drive.

FIG. 3(A) is a top view showing first embodiment of the magnetooptichead according to the present invention, and FIG. 3(B) is a side viewshowing the same.

A laser beam generated from a laser diode (not shown) is focused on thedisk as a spot through a condenser lens 101 supported by a condenserlens holder 102 made of a non-magnetic material such as resin. Fourpermanent magnets 103 are fixed to four side surfaces of the condenserlens holder 102, respectively, so as to be movable together with thecondenser lens 101.

One of the features of the present invention lies in the above fourmagnets 103 so mounted that the same four magnetic pole surfaces (Southpole in FIG. 3) are confronting each other. The function of thispermanent magnet arrangement is to apply an external magnetic field on adisk to record or erase information to or from the disk and to implementboth focusing and tracking operations of a laser beam irradiated throughthe condenser lens 101, as described later in more detail.

The permanent magnet 103 is made of a magnetic material of NdFeB groupwith a maximum energy product (BH)max=41MGOe and a reversibletemperature coefficient -0.071%/° C. The component of the magnet can beclassified into three categories, as ferromagnetic Fe rich-phase of Nd₂Fe₁₄ B pyramidal quadratic (tetragonal) system; non-ferromagnetic Rrich-phase of cubic system including 90 wt. % of substituent R(rare-earth element including yttrium Y) such as Nd_(97YFe), Nd_(ps),Fe; non-ferromagnetic B (Boron) rich-phase of pyramidal quadratic systemsuch as Nd₂ Fe₇ B₆ including oxide.

Under the condenser lens 101, a counterweight 104 is attached tocounterbalance the weight of the condenser lens 101. The condenser lensholder 102 is movably supported by two upper and lower rubbersuspensions 105 each having four leg portions 106 fixed to a frame 109.Further, a pair of focusing coils 107F are fixed to the frame 109 atsuch positions as to each come close to the outer flat surface of eachfocusing magnet 103F, extending in parallel to the outer flat surfacethereof. Furthermore, a pair of two tracking coils 108T are fixed to theframe 109 at such position as to each come close to the outer flatsurface of each tracking magnet 103T, also extending in parallel to theouter flat surface thereof.

With reference to FIG. 4, the feature of the present invention such thatthe head 5a or 5b is driven in the axial direction for focusing controland in the radial direction for tracking control will be describedhereinbelow, while applying a constant external magnetic field to thedisk for providing erasing or recording operation.

In operation, the disk 1 is rotated in a predetermined direction by themotor 2. The head 5a or 5b should be positioned a constant distance awayfrom the surface of the disk 1 to focus the laser beam guided throughthe condenser lens 101 mounted on the head to the disk. For doing this,focusing current is passed through the focusing coils 107F to generate amutual magnetic force generated between the two permanent magnets 103Fand the two focusing coils 107F in accordance with Fleming's left-handrule, so that the condenser lens 101 is moved in the axial directionthereof for focusing control operation against a elastic force of thetwo rubber suspensions 105.

On the other hand, the head 5a or 5b should be located at a designatedradial position of the disk to track a designated sector address on thebasis of the laser beam guided toward the disk through the condenserlens 101 mounted on the head. For doing this, tracking current is passedthrough the four tracking coils 108T to generate a mutual magnetic forcegenerated between the two permanent tracking magnets 103T and the fourtracking coils 108T in accordance with the same Fleming's rule, so thatthe condenser lens 101 is moved in the radial direction thereof fortracking control operation against another elastic force of the tworubber suspensions 105.

In addition to the above focusing and tracking operations, the fourpermanent magnets 103F and 103T serve to apply an external magneticfield as shown by dashed lines in FIG. 4 onto the disk 1 for permittinginformation recording or erasing operation. In other words, these fourpermanent magnets 103 are used for providing three magnetic fields offocusing, tracking, and recording or erasing operations. For these threeoperations, four permanent magnets 103 are arranged in such a way thatthe same magnet poles thereof are confronting each other and the samesize magnets are spaced at regular angular intervals around thecondenser lens 101, therefore, the magnetic flux flows from the fourouter flat surfaces of the four magnets, extending radially outward ofthe condenser lens 101 and returning axially inward thereof, to the fourinner flat surfaces thereof, as shown in FIG. 4.

In the head of the present invention, an intensity of the externalmagnetic field generated from the four magnets 103 is previously sodetermined as to be sufficient for information recording and erasingoperation under the condition that the laser beam from the head 5a or 5bis focused at a predetermined track cf the disk 1. In the prior arthead, the movable permanent magnets fixed with the condenser lens serveonly to move the condenser lens relative to the fixed coils, and anadditional permanent magnet is provided for applying an externalmagnetic field necessary for information recording and erasingoperations. Therefore, when dynamic axial runout occurs in the disk, theexternal magnetic field inevitably fluctuates.

On the other hand, in the present embodiment, since the permanentmagnets fixed to the condenser lens 101 are used in common for applyingan external magnetic field to the disk, the intensity of field requiredfor recording and erasing operation can automatically be kept constant(e.g. 400 Oe), even if dynamic axial runout occurs in the disk rotation,as long as the focusing operation is automatically being controlled.Therefore, it is possible to record or erase information stably to orfrom the disk.

Further, since the external magnetic field generating unit isincorporated in the lens actuator, the magnetooptic disk drive can beminimized in size or thickness and therefore the track access time canbe reduced because the head weight can be reduced markedly. Further, areduction in weight of the disk drive serves to further increase theaccess speed.

In a certain test using the head of the present invention, when a 1 MHz50% duty signal was recorded on a multi-layer TbCo disk under theconditions that the recording laser power is 5 mW; the disk rotationalspeed is 1,200 rpm; and the readout laser power is 1.5 mW, the C/N(carrier-to-noise ratio) (i.e. S/N ratio within a predeterminedbandwidth) is 50 dB or more for the 30 KHz bandwidth.

Further, in FIGS. 3 and 4, the same south magnetic poles are confrontingeach other. However, it is of course possible to arrange the magnets sothat the same north poles are confronted each other.

FIGS. 5(A) and (B) show a first modification of the first embodimentshown in FIG. 3. In these drawings, only two permanent magnets 103 arearranged close to the condenser lens 101. The intensity of the magneticfield generated by the permanent magnets 103 should be high enough toallow information to be recorded or erased on or from the disk andsufficient focusing and tracking drive sensitivities. In thismodification, a pair of coils are formed by bonding two tracking coils208T to a focusing coil 207F, respectively. Each of the bonded coils isarranged near the outer flat surface (North pole) of the magnet 103 inparallel thereto.

FIG. 6 shows a second modification of the first embodiment shown in FIG.3. In this drawing, the mutual positional relationship between the fourpermanent magnets 103 and four coils 107F and 108T are the same as shownin FIG. 3. The lens holders 302 are movably supported by four metalwires 305 fixed by a wire support block 309, so as to be movable in theaxial (focusing) and radial (tracking) directions of the condenser lens101. Further, a gel material 311 is injected to four holes to which thewires 305 are passed to provide dumping effect of the condenser lens101. That is, four wires 305 are used to support the condenser lens 101and the magnets 103 in place of the rubber suspensions 105 shown in FIG.3.

FIGS. 7(A) and (B) show a second embodiment of the magnetooptic headaccording to the present invention, in which only a single hollowfocusing coil 407F is provided. In the drawings, the condenser lens 101is supported by a cylindrical lens holder 402 made of a non-magneticmaterial Further, a hollow permanent magnet 403F is fixed to thecylindrical lens holder 402 coaxially with the optical axis of thecondenser lens 101. The cylindrical hollow magnet 403F is magnetizedradially from the outer circumference to the inner circumference thereofas shown in FIG. 7. The lens holder 402 and the permanent magnet 403Fare movably supported between two annular spring plates 405 forsuspending the movable lens 101 and the magnet 403F. The outermostcircumference of each of the annular spring plates 405 is fixed betweenan annular coil fixing frame 409 and one of two outer frames 410.

In this second embodiment, since the condenser lens 101 is moved up anddown only in the focusing direction and further, the cylindrical magnet403F is arranged throughout the outer circumference of the condenserlens 101, it is possible to obtain a strong external magnetic field bymeans of a relatively small permanent magnet for information recordingor erasing operation However, in this second embodiment, no trackingcoil is provided.

In the above-mentioned embodiments, it is preferable to disposehigh-permeability material such as pure iron near the focusing andtracking coils so as to shield the coils from the outside, extending inthe axial direction of the condenser lens, to increase the magnetic fluxdensity at the magnetic gaps, that is, to increase the focusing ortracking drive sensitivity.

As described above, in the magnetooptic head according to the presentinvention, since the permanent magnets are fixed to the condenser lensso as to be movable relative to the disk, the intensity of externalmagnetic field applied to the disk is always kept constant in theaccompany of the focusing control, thus improving the quality ofinformation signals to be recorded. Further, since the permanent magnetsare used only to drive the condenser lens for focusing and operation,the magnetooptic head can be reduced in size and in thickness withoutmagnetic interference with other magnets Furthermore, since the externalmagnetic field generating means is housed within the magnetooptic head,the disk can be loaded more readily into the disk drive.

FIGS. 8(A) and (B) show a third embodiment of the magnetooptic headaccording to the present invention, in which a pair of Hall elements arearranged to detect the moving direction and the movement distance of thecondenser lens from the neutral position in tracking operation.

In FIG. 8, the numeral 110 denotes a coil mounting plate to which pureiron plate 111 of high permeability is fitted to increase the magneticflux density between the magnet and the coil.

Further, two Hall elements HE bonded on two Hall element holders 501 aredisposed close to the magnetic pole surfaces (North pole in FIG. 8),respectively, along the tracking direction Y, in order to detect theY-directional displacement of the condenser lens 101. These Hallelements HE are disposed at the central position of the permanentmagnets 103, respectively.

The Hall element HE has four terminals as shown in FIG. 9(A), two ofwhich are voltage supply terminals and the remaining two of which areoutput terminals for outputting two Hall voltage signals indicative ofdetected magnetic field intensity. In FIG. 8, two Hall elements HE aredisposed so as to face the same North poles of the magnets 103.

FIG. 9(B) show a circuit for detecting the lens position on the basis ofthe output signals of the two Hall elements HE. A first Hall voltage e₁indicative of a neutral magnetic field position (central N-poleposition) of a first magnet 103(1) can be detected by obtaining adifferential level of two outputs A and B of a first Hall element HE(1).A second Hall voltage e₂ indicative of a neutral magnetic field position(central N-pole position) of a second magnet 103(2) can be detected byobtaining a differential level of two outputs A and B of the second Hallelement HE(2). Further, a lens position signal can be generated byfurther obtaining a differential level of these two Hall voltages e₁ ande₂.

Further, it is of course possible to detect the condenser lens positionby one Hall element and one differential amplifier.

During the tracking operation of the laser beam, when the lens positionsignal exceeds a predetermined level in voltage level, this signal isapplied to an external actuator such as a linear motor to move the wholehead, so that the displacement of the condenser lens in the radialtracking direction within the head is reduced by the linear motor into aneutral position of the condenser lens.

FIG. 9(C) shows the relationship between the differential output and thelens displacement, which indicates an excellent linearity over atracking movement range of ±400 μm. The detection sensitivity of theabove example is about 0.27 mV/μm.

FIG. 10 shows a first modification of the third embodiment of the headaccording to the present invention.

In FIG. 8, the focusing and tracking coils are formed into a flat shape,separately. In FIG. 10, however, each tracking coil 108T is wound arounda coil mounting plate 110 extending in the axial direction of the lens101 and additional each focusing coil 107F is wound over the trackingcoil 108T extending in the radial direction of the lens 101. Further,two Hall elements HE are fixed to the two Hall element holders 501,respectively, in the same way as in FIG. 8.

In the above first modification shown in FIG. 10, although the Hallelement HE is mounted or each Hall element holder 501, it is possible tofix the Hall element to any fixed elements relative to the movablemagnet 103 at such a position as to face the pole surface of the magnet.In practice, the Hall element can be fixed to the coil mounting plate110 or the tracking coil 108T, as shown in FIG. 11(A). In this case, twoHall elements HE are mounted on two focusing coils 107F, respectively,in the X-direction in FIG. 10. Further, it is also preferable to fix twoHall elements HE on the tracking coil 108T, as shown in FIG. 11(B), adistance spaced away from each other in symmetrical relationship withrespect to the pole surface of the magnet 103. In the case shown in FIG.11(B), the two Hall elements are mounted on either or both of the twofocusing coils 107F.

FIGS. 12 and 13 show some examples in which the Hall elements areincorporated in a head of axially slidable actuator type.

In FIG. 12(A), a condenser lens 601 is mounted on a non-magnetic lensholder 602. The lens holder 602 is slidably and pivotally supported by ashaft 604 via a cylindrical bearing 603. A cylindrical focus coil 605Fand four flat tracking coils 606T are fixed to the lens holder 602.These two coils 605F and 606T are movably disposed between twocircular-arc shaped permanent magnets 608 connected by a yoke 607.Therefore, when current is passed through the focus coil 605F, the lensholder 602 is moved up and down in the focus control direction. On theother hand, when current is passed through the tracking coil 606T, thelens holder 602 is pivoted clockwise or counterclockwise in the trackingdirection.

Further, other permanent magnets 609 are disposed on the side remotefrom the lens 601 via a rubber damper 612, as a counterweight forbalancing the weight of the condenser lens 601. And, a pair of Hallelements HE are fixed to two Hall element mounting plate 611 so as toface the pole surfaces of the magnets, respectively.

Therefore, a lens position signal can be generated by obtaining adifferential signal between two Hall sensor output levels. In thisexample, the permanent magnets 609 are used in common for thecounterweight against the condenser lens 601 and the magnetic fieldgenerating elements for the Hall elements for detecting lens position.

FIG. 13(A) shows another example, in which a cylindrical permanentmagnet 713 magnetized in the radial direction is fixed on the condenserlens 701 so as to apply magnetic fields to two Hall elements HE todetect the condenser lens position. Further, the numeral 709 denotes acounterweight against the condenser lens 701 and the magnet 713; thenumeral 712 denotes a rubber damper. The magnetic field from the magnets713 is also available as an external magnetic field required to recordor erase information to or from the disk.

In the above description, the shape and the magnetization direction ofthe permanent magnets arranged to detect the position of the condenserlens are not limited to the ones shown in FIGS. 8, 10, 11, 12 and 13.Further, either one of the magnetic poles N and S can be located so asto face the Hall element.

Furthermore, it is possible to use any kind of other magnetic fielddetecting elements such as magnetic resistance element (the resistancethereof changes according to the intensity of magnetic field), withoutbeing limited to Hall element.

The actuator provided with the condenser lens position detectingfunction according to the present invention is available for laser cardreading pickup apparatus as well as the pickup head of read-only,write-once, erasable types, etc.

In any cases where the Hall element detects lens displacement beyond apredetermined value, the lens position signal is outputted. In responseto the lens position signal, a linear motor for allowing coarse accessof the head to a designated track is actuated and further the trackingactuator of the head is locked by passing a current through the trackingcoil for prevention of lens vibrations, so that the head is smoothlyfine-adjusted to a designated track immediately after the head has beencoarse-adjusted and stopped at near the required track, thus reducingthe access time of the head to a track.

Furthermore, in the above description only the permanent magnets havebeen explained. However, it is possible to use any magnetic fieldgenerating means such as electromagnets.

According to the present invention, it is possible to provide alow-priced magnetooptic head provided with condenser lens positiondetecting function, without providing an additional external lensposition detecting apparatus which requires a relatively complicatedposition adjustment procedure.

The magnetooptic head according to the present invention has thefollowing advantages: (1) even when dynamic axial runout of the diskoccurs, since the external magnetic field intensity on the disk will notfluctuate due to laser beam focusing operation, it is possible to stablyrecord or erase information to or from the disk; (2) the permanentmagnets mounted on the head are used in common for external field, focusand tracking operations and condenser lens position detection (incooperation of magnetic field detecting elements); (3) since all thenecessary components are housed within head, the head can be minimizedin size and thickness; (4) when Hall elements are incorporated in thehead, the lens position can be detected for providing a high speed headaccess to a designated track.

What is claimed is:
 1. A magnetooptic head for recording/erasinginformation on/from a recording medium by irradiating the recordingmedium with a laser beam within a recording/erasing external magneticfield applied to the recording medium, which comprises:(a) a cylindricalcondenser lens for focusing the beam on the recording medium; (b) acylindrical hollow permanent magnet fixedly arranged outside saidcondenser lens coaxially therewith and magnetized radially from a centerthereof to an outer circumferential surface thereof; for generating arecording/erasing external magnetic field and focusing actuator magneticfield in a radial direction of said condenser lens on the basis of anaxial magnetic flux component thereof; (c) a cylindrical focusing coilarranged outside said cylindrical hollow permanent magnet coaxiallytherewith within the magnetic field generated by said cylindrical hollowpermanent magnet for moving said condenser lens and said cylindricalhollow permanent magnets together in the axial direction of therecording medium to provide a focusing servo operation of the laser beamin cooperation with a radial magnetic component of said permanentmagnet; and (d) an elastic member for movably supporting said condenserlens and said permanent magnet together relative to said focusing coilin the axial direction of said recording medium.